The present invention relates to the general field of turbine blades, and, more particularly, to the shape of the cooling air evacuation slots in the trailing edges of rotor or stator blades of a turbomachine turbine.
A turbomachine turbine (e.g. a high-pressure turbine) consists of a plurality of stages each made up of a stator nozzle and a rotor wheel. The turbine nozzle has a plurality of stator blades designed to redirect the gas stream going through it, and the rotor wheel of the turbine is constituted by a plurality of moving blades.
The rotor and stator blades of such a turbine are subjected to the very high temperatures of the gases coming from the combustion chamber and passing through the turbine. These temperatures reach values that are much higher than those which the blades that are in contact with the gas can withstand without being damaged, thereby shortening their lifetime.
In order to reduce the damage said hot gases cause the blades, there exist systems that provide the blades with internal cooling circuits for reducing the temperature of said blades. Using such circuits, the cooling air that is introduced into a blade passes through said blade along a path formed by cavities made in the blade before being expelled via slots that open out in the surface of the blade, between the blade base and the blade tip.
Unfortunately, for a moving turbine blade, it has been found in practice that the slot nearest to the blade base is not properly cooled. In addition, for a stator turbine blade, the slots nearest to the blade base and the blade tip are also not properly cooled. In fact, cracks tend to form in the trailing edge of the blade, in the vicinity of said slots. Such cracks jeopardize the lifetime of the blade, in particular, by decreasing its strength.
FIG. 7 shows where such cracks appear in a moving turbine blade. This figure is a fragmentary perspective view of a moving blade 100 of a high-pressure turbine. The blade 100 has an airfoil 102 that is connected at the blade base 104 to a platform 106 via a connection zone 108. The airfoil 102 of the blade extends axially between a leading edge (not shown in the figure) and a trailing edge 110. In order to cool the blade 100, air moves over said blade following a path formed by cavities (not shown) made inside the blade, before being expelled via evacuation slots 112 that open out in the airfoil 102 of the blade, at its trailing edge 110.
Each evacuation slot 112 is formed, in particular, by an end wall 114 provided with an opening (not shown) that opens into the cavities through which the cooling air flows. Each slot also has a setback wall 116 extending from the end wall 114 to the trailing edge 110 of the blade, and a top wall 118 and a bottom wall 120 that extend between the setback wall 116 and the airfoil 102 of the blade.
In practice, it has been found that one or more cracks 122 (a single crack is shown in the figure) form at the evacuation slot 112a that is nearest to the platform 106 (referred to below as the “bottom” slot). More precisely, cracks 122 form in the setback wall 116 of the bottom slot 112a and propagate axially from the trailing edge 110 of the blade towards the end wall 114 of the slot.
Such cracks arise mainly from a high concentration of stress in the bottom slot 112a, said stress being caused, in particular, by the bottom wall 120 of said bottom slot. There is the risk that such cracks may propagate over the entire airfoil 102 of the blade, thereby reducing its lifetime.
For a stator turbine blade, identical cracks appear both at the evacuation slot nearest to the platform disposed beside the blade base, and at the evacuation slot nearest to another platform connected to the blade at its tip (referred to below as the “top” slot).
In order to prevent cracks from appearing, U.S. Pat. No. 6,062,817 suggests, for a moving turbine blade, eliminating a portion of the bottom wall of the evacuation slot nearest to the platform, so that a portion of the setback wall of said slot extends radially between the top wall and the platform of the blade.
Nevertheless, that solution is insufficient. In fact, the bottom slot of the blade of that patent still has sharp edges on its bottom wall. The resulting sudden change in thickness prevents the cooling air evacuated via said slot from flowing properly. Thus, the evacuated air can no longer cool the connection zone between the platform and the blade base, so cracks that are particularly detrimental to the lifetime of the blade appear in that zone.